1. Field of the Invention
The present invention generally relates to integrated circuit structures and, more particularly, to multilayer interconnection structures for VLSI devices.
2. Description of the Related Art
Thin films of aluminum and aluminum alloys are typically used as conducting materials to form interconnections between components and devices in integrated circuits. Aluminum's low resistivity, low cost, low weight and ability to adhere strongly to silicon and silicon oxide surfaces common in silicon-based semiconductor devices are some of the material's advantages in semiconductor fabrication.
As the dimensions of interconnects have decreased to the sub-micron level, failures of interconnects have increased for a variety of reasons, such as electromigration and stress-induced migration. Electromigration failures occur when ions of the conductor material migrate and leave voids in the conductor material. Voids in the conductor material can also occur during subsequent processing steps, such as subsequent metallization or insulation steps. Stress-induced cracks and voids in conductor materials can also lead to failures in integrated circuits.
Many three layer metallization structures have been used to form thin interconnects while attempting to prevent formation of voids within the interconnects. It should be understood that interconnects are typically disposed on dielectric material, except where the interconnects form an electrical contact with a plug or via, which is typically made of a conductive material. Therefore, the lower layer of the metallization structure should be capable of forming an adequate electrical connection with the conductive material used as a contact. As a first example, a three layer metallization structure has been used that contains titanium as the lower layer, aluminum (perhaps containing about 5% copper) as the middle layer, and titanium nitride as the top layer has been used. Since titanium reacts well with tungsten, it is a good choice for the bottom layer of the metallization stack. However, the middle layer of aluminum tends to react with the titanium to form TiAl3. This reaction can form a void in the interconnect and, thus, cause an open circuit failure.
As a second example, a three layer metallization structure has been used where the structure includes a lower layer of titanium, a middle layer of aluminum or aluminum alloy, and a top layer of titanium. However, this structure suffers from the same problem mentioned in the first example. Specifically, the titanium and aluminum can react to form TiAl3 and, thus, create a void in the interconnect.
By way of a third example, a three layer metallization stack having a bottom layer of titanium nitride, a middle layer of aluminum or aluminum alloy, and a top layer of titanium nitride has been used in an effort to avoid the problem mentioned above. However, this metallization structure does not contain a lower titanium layer to make a good electrical contact with a tungsten plug or a lower metallization layer of aluminum or tungsten.
Other multilayer conductors have been fabricated with the objective of improving the electrical and mechanical properties of aluminum interconnects. For example, a six layer interconnect structure of, from top to bottom, titanium nitride/aluminum-1% silicon-0.5% copper/titanium nitride/aluminum-1% silicon-0.5% copper/titanium nitride/titanium is said to provide improved electrical and mechanical properties. However, such complex multilayer conductors have many manufacturing disadvantages. In this example, it is difficult to form a six layer metallization structure on production wafers because of slow throughput. Also, the aluminum-silicon layers may for silicon precipitates during subsequent thermal processes that are difficult to remove during subsequent etches. Accordingly, a need exists for simpler multilayer conductors having the desired electrical and mechanical properties for use in integrated circuits.